Afghanistan's growing security forces aren't yet able to protect the entire nation and will require significant U.S. support for the foreseeable future, according to a new Pentagon report.

WASHINGTON—Afghanistan's growing security forces aren't yet able to protect the entire nation and will require significant U.S. support for the foreseeable future, according to a new Pentagon report that bolsters the argument for a long-term American presence in the country.

While the Afghan forces are holding their own in battles with the Taliban, pervasive government corruption "poses a major threat" that could undermine...